For years it has been difficult to combine existing types of heat, such as hot water (hydronic), with solar heat. Complex and expensive systems have been suggested but very few have been proven. Most prior attempts used anti-corrosion and anti-freeze materials to protect heat collectors, heat exchangers, and other parts. Those experimental systems required many expensive valves, heat exchangers, pumps, and so on. Some systems used liquid under pressure, subject to spring leaks, ruin solar collectors, poison drinking water with anit-freeze, and so on. Expensive heat exchangers were necessary. Those heat exchangers lowered efficiency of the systems.
There appeared to be no way to construct a simple low-cost system. There appeared to be no way to convert existing systems to solar heat, except at very high cost, with liklihood of failure and low efficiency, and with the possibility of poisoning drinking water.